when i was diagnosed on 11th April '17, I had 3 tumours, 10mm, 15mm & 25mm multifocal, mixed lobular & ductal malignant carcinomas.
Surgery was 23rd May '17 and they found 2 carcinomas about the same size (nurses said that the 2 smaller may have grown & joined up)- on my paperwork it had me as grade 2.
As I had 17 positive nodes (all of them) my stage is 3C, category 3, - when staging the amount of positive nodes seem to take precedence over the sizes of the carcinomas, however it makes sense to stage it on the largest otherwise.
These charts may help you make some sense of it. -
https://breast-cancer.canceraustralia.gov.au/diagnosis/stages-breast-cancerhttp://www.breastcancer.org/symptoms/diagnosis/cell_grade
I hope this helps somewhat